1. Field of the Invention
The present invention relates to a semiconductor laser element and a method of fabricating the same, and more particularly to a AlGaAs-based ridge-stripe semiconductor laser element withlow operational voltage and low element resistance, and a method of fabricating the element.
2. Description of the Related Art
In recent years, AlGaAs-based infrared wavelength semiconductor laser elements are used as light sources for reading devices, rewriting devices and initializers for optical disks. In particular, infrared high-output semiconductor laser elements of a broad-stripe type capable of oscillating lateral multi-mode laser light are expected as light sources for exciting solid lasers such as Nd:YAG and Nd:YVO4 having absorption band of the crystals at around 808 nm. Also processing tools such as welders based on use of the infrared high-output semiconductor lasers are becoming increasingly popular.
In these fields, the high-output semiconductor laser elements have particularly been on demand for realizing a still higher light confinement efficiency and a still lower threshold current.
Referring to FIG. 10, a configuration of a conventional AlGaAs-base semiconductor laser element having a buried-ridge-type structure will be described. FIG. 10 is a cross-sectional view of a conventional AlGaAs-base semiconductor laser element having the buried-ridge-type structure.
As shown in FIG. 10, a conventional buried-ridge-type AlGaAs-base semiconductor laser element 200 includes a double-hetero-stacked structure formed on an n-GaAs substrate 201, where the stacked structure comprises an n-GaAs buffer layer 202, an n-Al0.47Ga0.53As cladding layer 203, an active layer section 204, a p-Al0.47Ga0.53As first cladding layer 205, an etching stop layer 206, a p-Al0.47Ga0.53As second cladding layer 207 and a p-GaAs contact layer 208, all of which are epitaxially grown sequentially in this order.
The p-second cladding layer 207 and p-contact layer 208 are formed as a ridge so as to constitute a current injection region 220. Both lateral sections of the ridge composing the current injection region 220 are filled with an n-GaAs current blocking layer 211 to thereby form non-current-injection regions 221.
A p-side electrode 212 is formed on the upper surfaces of the p-contact layer 208 and the n-GaAs current blocking layer 211, and an n-side electrode 213 is formed on the back surface of the n-GaAs substrate 201.
In the semiconductor laser element 200, filling of both lateral sections of the ridge-stripe-patterned current injection region 220 with a semiconductor material of a conduction type opposite to the current injection region 220 is successful in realizing narrowing of both the current and refractive-index-based waveguide at the same time.
It can thus be said that the aforementioned semiconductor laser element 200 has a configuration capable of effectively confining both injected carriers and laser light.
The following paragraphs will describe a method of fabricating the conventional semiconductor laser element 200 referring to FIG. 11A to FIG. 13F. FIGS. 11A and 11B, FIGS. 12C and 12D, and FIGS. 13E and 13F are cross-sectional views showing layer structures in the individual process steps in the fabrication of the conventional semiconductor laser element 200.
First, as shown in FIG. 11A, the n-GaAs buffer layer 202, the n-Al0.47Ga0.53As cladding layer 203, the active layer section 204, the p-Al0.47Ga0.53As first cladding layer 205, the etching stop layer 206, the p-Al0.47Ga0.53As second cladding layer 207, and the p-GaAs contact layer 208 are epitaxially grown sequentially in this order on the n-GaAs substrate 201 in the first epitaxial growth step by an organometallic vapor phase growth process such as the MOVPE process and MOCVD process, to thereby form a stacked structure 210 having a double hetero-structure.
In the epitaxial growth, Si, Se and so forth are used as the n-type dopant, and Zn, Mg, Be and so forth as the p-type dopant.
Next, as shown in FIG. 11B, an SiO2 film 214 is formed on the top surface of the stacked structure 210, that is, the upper surface of the p-GaAs contact layer 208, by a CVD (Chemical Vapor Deposition) process or the like, and further on the SiO2 film 214, a stripe-patterned resist mask 215 is formed by photolithography.
Next, the SiO2 film 214 is mask-patterned with the resist mask 215, and the resist mask 215 is then removed, to thereby form an SiO2 mask 214 on the p-GaAs contact layer 208, as shown in FIG. 12C.
Next, the p-GaAs contact layer 208 and the p-Al0.47Ga0.53As second cladding layer 207 are etched by wet etching technique under masking with the SiO2 mask 214, to thereby form a ridge.
The etching is carried out using an etchant which is capable of completely removing the p-GaAs contact layer 208 and the p-Al0.47Ga0.53As second cladding layer 207, and having an etching selectivity enough to terminate the etching on the surface of the etching stop layer 206. This makes it possible to selectively remove the p-Al0.47Ga0.53As second cladding layer 207 without affecting the etching stop layer 206.
Next, as shown in FIG. 13E, the process advances to a second epitaxial step, where the n-GaAs current blocking layer 211 is grown on both lateral potions of the ridge. Because the SiO2 mask 214 resides on the ridge, the GaAs current blocking layer 211 does not grow on the ridge.
Next as shown in FIG. 13F, the SiO2 mask 214 is removed, the p-side electrode 212 is formed on the p-contact layer 208 and the n-GaAs current blocking layer 211, and the n-side electrode 213 is formed on the back surface of the n-GaAs substrate 201.
The aforementioned conventional semiconductor laser element 200 is obtained after all of the above-mentioned process steps.